1. Field of the Invention
The present invention relates generally to compensation for chromatic dispersion and nonlinearity in optical fiber communication, and more particularly to a method for optical fiber communication which can compensate for chromatic dispersion and nonlinearity to allow long-haul transmission, and a terminal device and system for use in carrying out the method.
2. Description of the Related Art
Owing to recent developments of low-loss silica optical fibers, various optical fiber communication systems each using such an optical fiber as a transmission line have been put to practical use. The optical fiber itself has a very wide band. However, a transmission capacity by the optical fiber is actually limited by a system design. The most important limitation is due to waveform distortion by chromatic dispersion occurring in the optical fiber. Further, the optical fiber attenuates an optical signal at a rate of about 0.2 dB/km, for example. Loss of the optical signal due to this attenuation has been compensated for by adopting an optical amplifier such as an erbium doped fiber amplifier (EDFA) that is a typical example. The EDFA has a gain band in a 1.55 xcexcm band where a silica optical fiber gives a lowest loss.
The chromatic dispersion that is often referred to simply as dispersion is a phenomenon such that the group velocity of an optical signal in an optical fiber changes as a function of the wavelength (or frequency) of the optical signal. In a standard single-mode fiber, for example, an optical signal having a longer wavelength propagates faster than an optical signal having a shorter wavelength in a wavelength region shorter than 1.3 xcexcm, and the resultant dispersion is usually referred to as normal dispersion. In this case, the dispersion (whose unit is ps/nm/km) takes on a negative value. In contrast, an optical signal having a shorter wavelength propagates faster than an optical signal having a longer wavelength in a wavelength region longer than 1.3 xcexcm, and the resultant dispersion is usually referred to as anomalous dispersion. In this case, the dispersion takes on a positive value.
In recent years, the nonlinearities of an optical fiber have received attention in association with an increase in optical signal power due to the use of an EDFA. The most important nonlinearity that limits a transmission capacity is an optical Kerr effect occurring in an optical fiber. The optical Kerr effect is a phenomenon such that the refractive index of an optical fiber changes with the power or intensity of an optical signal.
A change in the refractive index modulates the phase of an optical signal propagating in an optical fiber, resulting in the occurrence of frequency chirping which changes a signal spectrum. This phenomenon is known as self-phase modulation (SPM). There is a possibility that such a change in spectrum due to SPM may further enlarge the waveform distortion due to chromatic dispersion.
In this manner, the chromatic dispersion and the optical Kerr effect impart waveform distortion to an optical signal with an increase in transmission distance. Accordingly, to allow long-haul transmission by an optical fiber while ensuring a transmission quality, the chromatic dispersion and the nonlinearity must be controlled, compensated, or suppressed.
It is therefore an object of the present invention to provide a method for optical fiber communication which can compensate for chromatic dispersion and nonlinearity to allow long-haul transmission, and to also provide a terminal device and system for use in carrying out the method.
In accordance with an aspect of the present invention, there is provided a method for optical fiber communication. First, a device for outputting an optical signal having a variable optical power to an optical fiber transmission line is provided. Secondly, the optical signal transmitted by the optical fiber transmission line is converted into an electrical signal. Thirdly, a parameter (e.g., a bit error rate or an eye opening) related to waveform degradation of the electrical signal is detected. Finally, the optical power of the optical signal to be output to the optical fiber transmission line is controlled according to the parameter detected so that the waveform degradation of the electrical signal is improved.
In general, nonlinearity of an optical fiber or a nonlinear phenomenon occurring in an optical fiber used as the optical fiber transmission line is dependent on the optical power of an optical signal to be output to the optical fiber. In the method of the present invention, the optical power of the optical signal is changed according to the state of the optical fiber transmission line, e.g., according to the kind of an optical fiber used as the optical fiber transmission line, thereby controlling the nonlinearity of the optical fiber. Therefore, long-haul transmission is allowed with a transmission quality being ensured by compensation for chromatic dispersion and nonlinearity.
In accordance with another aspect of the present invention, there is provided a system comprising first and second terminal devices and an optical fiber transmission line connecting the first and second terminal devices. The first terminal device comprises an optical transmitter for outputting an optical signal having a variable optical power to the optical fiber transmission line. The second terminal device comprises an optical receiver for converting the optical signal transmitted by the optical fiber transmission line into an electrical signal, a monitor unit for detecting a parameter related to waveform degradation of the electrical signal, and means for transmitting supervisory information on the parameter detected to the first terminal device. The first terminal device further comprises a control unit for controlling the optical power according to the supervisory information so that the waveform degradation of the electrical signal is improved.
In accordance with a further aspect of the present invention, there is provided a terminal device comprising an optical transmitter for outputting an optical signal having a variable optical power to an optical fiber transmission line; means for receiving supervisory information on a parameter detected in relation to waveform degradation of the optical signal transmitted by the optical fiber transmission line; and means for controlling the optical power according to the supervisory information so that the waveform degradation of the optical signal is improved.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.